Authorities say a loud explosion has leveled a home in Indianapolis and set four others ablaze. / Matt Kryger, AP

by John Tuohy, The Indianapolis Star

by John Tuohy, The Indianapolis Star

INDIANAPOLIS -- Law enforcement officials said Tuesday they suspect gas was intentionally released into a home before a spark caused it to explode, setting off a deadly chain of blasts in the city's Richmond Hill subdivision that killed two people.

Authorities said the fumes seeped from a manipulated pipe or stove and gathered within the house, until a small spark ignited it.

"The place was full of gas," a source close to the investigation told The Indianapolis Star, speaking on the condition of anonymity.

A second law enforcement source said that remote detonation is the leading theory now of how the blast occurred, but more investigation is needed before they can be sure.

Neither source would say what evidence led them in that direction.

On Monday, prosecutors said they were hoping to talk with anyone who had information about a white van and its occupants seen in the neighborhood on the afternoon of the explosion.

Police confirmed Tuesday that they had taken the van and inspected it, but they released no other details such as who owned the van.

As for a gas explosion, experts said the spark could be activated from outside the house by using a remote control to turn on an electrical appliance.

"All it needs is a tiny spark, like when a light, a television or a thermostat is turned on," said Jay A. Siegel, a forensic and investigative science consultant who taught at Indiana University-Purdue University, Indianapolis. "It's entirely possible, though I've never heard of anyone actually doing it."

Officials had said Monday that the explosion is now a homicide investigation, meaning someone was directly responsible for the two deaths. The law enforcement sources would not say whether authorities had zeroed in on a particular suspect but said the person had at least a rudimentary understanding of how to cause explosions.

Public Safety Director Troy Riggs said at an unrelated news conference Tuesday that the investigation was "proceeding."

"Hopefully we will get the person or persons responsible for this and are working closely with the prosecutor," Riggs said. "It is troubling that this is a criminal act and not an act of God."

The detonation would come after the concentration of the gas in the house reached 10 percent, Siegel said, and the fastest way to get those levels would be to unscrew a gas line leading into the house or an appliance.

"Ten percent gas and 90 percent oxygen is ideal," Siegel said.

Bureau of Alcohol Tobacco, Firearms and Explosives spokesman Dave Caulson said he had no knowledge of the home being booby trapped.

"There has been no discussion amongst myself and investigators like that," he said. Caulson said ATF is playing a secondary role in the investigation, which is being led by the Indianapolis fire and police departments.